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Author Topic: Proper fuse-circuit breaker size for branch circuits.  (Read 4685 times)

John Roberts {JR}

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Re: Proper fuse-circuit breaker size for branch circuits.
« Reply #10 on: September 15, 2016, 10:32:50 am »

You should probably start with the 15s unless you know that the ENTIRE circuit is wired with 12awg.  If you blow a 15A fuse on a circuit wired with 14ga wire then fix the over-current.  Much less likely to have a house fire that way.
I bought one package of 15A fuses at the same time and will experiment with using them. I have been looking at my appliances to think about what may have caused fuses to open in the past.

For several years I used a 115V in-wall air conditioner in my main room, even thought there was a 230V outlet for it (brain fart). That air conditioner probably dumped a bunch of current into the same branch as my kitchen so using my early microwave oven at full tilt, or the coffee maker added to the air conditioner baseline might have exceeded the fuse limit. I have never purchased a 30A fuse myself, but there are a pile of them out by my fuse box so I used them rather than go without electricity. I bought some 20A fuses a couple years ago, but didn't bother to remove the 30A fuses. Over the years I developed the habit of not running my coffee grinder and electric kettle at the same time. Apparently that took out fuses in the past.

Thinking back I've lost a few fuses from the window air conditioner in my back bedroom. I haven't even turned it on this summer thanks to no 100'+ heat waves this year,,, When it's really hot I need to run both air conditioners.

I suspect the newer 230V heat pump/air co in place of the old 115V in-wall unit has reduced a bunch of extra current load from my kitchen branch.   

JR
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Art Welter

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Re: Proper fuse-circuit breaker size for branch circuits.
« Reply #11 on: September 19, 2016, 11:52:22 am »

I bought one package of 15A fuses at the same time and will experiment with using them. I have been looking at my appliances to think about what may have caused fuses to open in the past.

Thinking back I've lost a few fuses from the window air conditioner in my back bedroom. I haven't even turned it on this summer thanks to no 100'+ heat waves this year,,, When it's really hot I need to run both air conditioners.

I suspect the newer 230V heat pump/air co in place of the old 115V in-wall unit has reduced a bunch of extra current load from my kitchen branch.   

JR, having recently moved in to a rental house I was faced with a similar deal- a 30 amp fuse in the carport blew the first time I ran my air compressor. I went to the hardware store and replaced it, and put a power strip with a 15 amp breaker on the outlet to prevent having to climb into the attic to replace the fuse in the future.

It turns out the 30 amp fuse was in series with a 20 amp breaker that had been wired in a year or two ago when the landlord upgraded the main panel to power a new (very used..) 220 volt central/heat/air conditioner and separate heat pump.
I had brought it (along with around 19 other code violations..) to the attention of the landlord, as I feared the 14 AWG wire the 30 amp fuse was on may have been wired into a large breaker, but discovered it was on the last breaker in the box (labeled "Dinning room ;^) ) when doing further testing a few days ago.

Since the time constant on the usual 30 amp fuse is much shorter than a usual 20 amp breaker, which can take 10 times the nameplate rating for a short term load like a motor starting, it can "blow" while the breaker never trips, until a short circuit happens, or the long term amperage rating is exceeded, usually by a combination of resistive loads like space heaters and lighting.

At any rate, good luck- I'd suggest purchasing some screw-in fuse replacing breakers if you don't want to bother putting in a new panel, though they probably cost more than a little Square D "Hom" panel loaded with breakers.

Art

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John Roberts {JR}

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Re: Proper fuse-circuit breaker size for branch circuits.
« Reply #12 on: September 19, 2016, 12:29:00 pm »

yup, my growing pile of removed 30A fuses are all fast blow while all the 15A and 20A fuses are slow-blow.

I wonder if a slow blow 20A would survive your compressor start up current spike? My (very old) package of 30A fuses said not for use in motor circuits (FWIW).

I haven't had any issue in recent years with fuses blowing, except for one incident of cowboy electrical work when I tried to replace an outlet while the branch was hot (do what I say not what I do).

Over the next few months I will experiment with replacing the 20A fuses with 15A to find where my weak links are. I expect my carpet shampooer will add some marginal load when I fire it up again this winter.

I may consider screw in breakers after I get this sorted if I need them but screw fuses are widely available, and I do not blow many (any) fuses these days.

JR 
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Art Welter

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Re: Proper fuse-circuit breaker size for branch circuits.
« Reply #13 on: September 19, 2016, 03:54:28 pm »

yup, my growing pile of removed 30A fuses are all fast blow while all the 15A and 20A fuses are slow-blow.

I wonder if a slow blow 20A would survive your compressor start up current spike? My (very old) package of 30A fuses said not for use in motor circuits (FWIW).
JR,

Air compressors are a often a particularly tough load to handle- the motor starts draws the usual big capacitor inrush, then high current while it spools up, and then as the air tank starts developing more pressure, the motor is put under progressively more load. When it knocked out the 30 amp fuse (which may have been "tired") it was just about to hit 90 PSI, where the automatic shut-off would have disconnected the motor.

My experience with fuses is that "fast" always seems to blow too soon, and "slow" may not provide the protection you want, though growing up I don't remember my Dad's air compressors or table saw ever taking out fuses.

I do remember as a 9 or 10 year old blowing the 15 amp fuse that fed every outlet and light in our basement. I had wired a remote control "robot" using two record player motors that trailed 20 feet of lamp cord around, the basement, after I figured out how to not wire the two switches as short circuits ;^).  Had to buy a spare fuse with the next week's paper route money so the missing spare would not give my "learning experience" away, I had already spent my pay on 40 feet of cord and two switches.

Art
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Keith Broughton

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Re: Proper fuse-circuit breaker size for branch circuits.
« Reply #14 on: September 19, 2016, 04:01:30 pm »

yup, my growing pile of removed 30A fuses are all fast blow while all the 15A and 20A fuses are slow-blow.

I wonder if a slow blow 20A would survive your compressor start up current spike? My (very old) package of 30A fuses said not for use in motor circuits (FWIW).

I haven't had any issue in recent years with fuses blowing, except for one incident of cowboy electrical work when I tried to replace an outlet while the branch was hot (do what I say not what I do).

Over the next few months I will experiment with replacing the 20A fuses with 15A to find where my weak links are. I expect my carpet shampooer will add some marginal load when I fire it up again this winter.

I may consider screw in breakers after I get this sorted if I need them but screw fuses are widely available, and I do not blow many (any) fuses these days.

JR
Before I wired the bedroom AC unit to a separate cct, my wife would blow the screw in fuses with the hair dryer.
Found some re-settable screw in fuses that proved quite handy.
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John Roberts {JR}

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Re: Proper fuse-circuit breaker size for branch circuits.
« Reply #15 on: September 19, 2016, 05:54:48 pm »

JR,

Air compressors are a often a particularly tough load to handle- the motor starts draws the usual big capacitor inrush, then high current while it spools up, and then as the air tank starts developing more pressure, the motor is put under progressively more load. When it knocked out the 30 amp fuse (which may have been "tired") it was just about to hit 90 PSI, where the automatic shut-off would have disconnected the motor.

My experience with fuses is that "fast" always seems to blow too soon, and "slow" may not provide the protection you want, though growing up I don't remember my Dad's air compressors or table saw ever taking out fuses.

I do remember as a 9 or 10 year old blowing the 15 amp fuse that fed every outlet and light in our basement. I had wired a remote control "robot" using two record player motors that trailed 20 feet of lamp cord around, the basement, after I figured out how to not wire the two switches as short circuits ;^).  Had to buy a spare fuse with the next week's paper route money so the missing spare would not give my "learning experience" away, I had already spent my pay on 40 feet of cord and two switches.

Art
OK now I'm confused... in your earlier post you say you bought a power strip with 15A breaker for that outlet. Is the compressor that blew the 30A fuse hanging with 15A breaker outlet strip?

What else was on that branch..? Yes fuses can get tired (fatigue) from too many near death experiences (thermal cycles).

JR

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Don't tune your drums half-ass. Listen to what a properly "cleared" drum sounds like.   http://circularscience.com/

Stephen Swaffer

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Re: Proper fuse-circuit breaker size for branch circuits.
« Reply #16 on: September 20, 2016, 01:01:42 pm »

Another cause for fuses blowing too soon is a bad connection near the fuse.  Bad connections cause heat and fuses are thermal devices.  I have repaired more than one case of requent fuse blowing by tightening wires or, in the case of cartridge fuses, cleaning and tightening the holders.
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Steve Swaffer

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Re: Proper fuse-circuit breaker size for branch circuits.
« Reply #16 on: September 20, 2016, 01:01:42 pm »


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